The present invention relates to a communication module, and particularly to a technology effective for application to communication modules (such as a transmitting module and a receiving module) suitable for use in an optical interconnect communication system.
An attempt has been made to transmit data within or between apparatuses such as an exchanger, a parallel computer, a general-purpose large computer, etc. through the use of an optical interconnect communication system. Several transmission modes or systems are considered as for the optical interconnect communication system. However, a parallel data transmission mode capable of achieving simplification of the system, a size reduction in the system, etc. has been considered to be promising.
The optical interconnect communication system of the parallel data transmission mode principally includes a transmitting module for converting electric signals to their corresponding light signals, an optical fiber array cable, and a receiving module for converting the light signals to their corresponding electric signals. The transmitting module has a semiconductor chip (hereinafter called xe2x80x9ctransmitting IC (Integrated Circuit)xe2x80x9d) in which channels for allowing signal converting means to convert voltage signals inputted from input terminals to current signals and outputting the same from output terminals, respectively are arranged in parallel in plural form, a light-emitting device array unit in which laser diodes for respectively converting the electric signals outputted from the output terminals of the transmitting IC to light signals are arranged in parallel in plural form, and a lens array unit in which micro lenses are arranged in parallel in plural form. The transmitting module takes a structure wherein these are accommodated or held in a metal package. The receiving module includes a lens array unit wherein micro lenses are arranged in parallel in plural form, a photoreceptor or light-detecting device array unit wherein photodiodes for respectively converting light signals to electric signals are arranged in parallel in plural form, and a semiconductor chip (hereinafter called xe2x80x9creceiving ICxe2x80x9d) wherein channels for respectively converting current signals inputted from input terminals to voltage signals through the use of signal converting means and outputting them from output terminals are arranged in parallel in plural form. The receiving module has a structure wherein these are held in a metal package.
On the other hand, since each of the transmitting IC and the receiving IC takes a multi-channel circuit configuration in the case of the parallel data transmission mode, crosstalk is apt to occur between adjacent channels. Therefore, the technology of reducing the crosstalk developed between the adjacent channels in the transmitting IC to xe2x88x9221 [dB] has been described in xe2x80x9cICD issued by Institute of Electronics, Information and Communication Engineers, ICD96-116, p.47-54 xe2x80x9c1.4 Gb/sx12-channel LD Driver IC for Optical Interconnectionxe2x80x9d.
In the transmitting IC, however, those other than light-emitting device drive circuits for respectively driving the laser diodes are held at a problem-free level because they are digitally operated, whereas in the case of the implementation of the receiving IC, a crosstalk of about 80 [mV] occurs between the adjacent channels if 800 [mV] is taken as internal logic amplitude. Since the receiving IC handles a small signal converted to a current by each photodiode, the crosstalk developed between the adjacent channels becomes critical. Assuming that the input impedance of an input preamplifier circuit used in a first stage is 1 [kxcexa9], for example, the crosstalk of 80 [mV] becomes equivalent to an input noise current of 80 [xcexcA].
On the other hand, since several tens of [xcexcA] are commonly used as the minimum signal input current, the crosstalk developed between the adjacent channels leads to fatality. According to the above-described reference, it describes that if the channels are spaced 1 [mm] from each other, then the crosstalk developed between the adjacent channels is reduced to near 30 [dB]. Even in this case, however, the receiving IC will suffer a crosstalk noise of about 25 [xcexcA], and hence a system margin is significantly reduced.
On the other hand, the technology of interposing a buffer area or zone for inhibiting the motion of carriers between each of memory cells and its corresponding switching circuit with a view toward preventing damage of memory information due to a parasitic bipolar element in an SRAM using a Bi-CMOS (Bipolar-Complementary Metal Oxide Semiconductor) technology is disclosed in Japanese Unexamined Patent Publication No. Sho 62-71265. The present technology can be applied even to a problem of crosstalk developed between adjacent channels in transmitting and receiving modules employed in an optical intercorrect communication system. A certain degree of effect can be expected by providing a buffer zone between an input preamplifier circuit and a digital circuit portion. However, crosstalk developed through a substrate cannot be ignored at a high frequency. Since no countermeasures are taken against the substrate, a sufficient effect cannot be achieved.
An object of the present invention is to provide a technology capable of reducing channel-to-channel crosstalk.
Another object of the present invention is to provide a technology capable of increasing the reliability of a communication module.
The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
A summary of typical ones of the inventions disclosed in the present application will be described in brief as follows:
(1) There is provided a communication module comprising:
a semiconductor chip in which channels for allowing signal converting means to convert current signals inputted from input terminals to voltage signals and outputting the same from output terminals respectively are arranged in parallel in plural form; and
wherein the semiconductor chip is comprised principally of a semiconductor substrate in which a second semiconductor layer is provided on a first semiconductor layer with an insulating layer interposed therebetween,
the signal converting means is formed in a channel forming region of the second semiconductor layer, which is defined for each channel,
the input and output terminals are formed on the channel forming regions of the second semiconductor layer with the insulating layer interposed therebetween, and
the first semiconductor layer is formed with resistivity smaller than that of the second semiconductor layer and fixed to a fixed potential.
(2) In the communication module described in the means (1), a conductive layer electrically separated from the channel forming regions and fixed to a reference potential is provided between the channel forming regions of the second semiconductor layer.
(3) In the communication module described in the means (1), each signal converting means referred to above has an input preamplifier circuit which performs an analog operation, and an internal circuit and an output circuit both of which are digitally operated,
the input preamplifier circuit is formed in an input circuit forming portion of the channel forming region of the second semiconductor layer,
the internal circuit is formed in an internal circuit forming portion of the channel forming region of the second semiconductor layer,
the output circuit is formed in an output circuit forming portion of the channel forming region of the second semiconductor layer, and
the input circuit forming portion of the channel forming region of the second semiconductor layer is enclosed on all sides by a conductor electrically isolated from each channel forming region referred to above and fixed to the fixed potential.
(4) In the communication module described in the means (1), the signal converting means referred to above has an input preamplifier circuit which performs an analog operation, and an internal circuit and an output circuit both of which are digitally operated,
the input preamplifier circuit is formed in an input circuit forming portion of the channel forming region of the second semiconductor layer,
the internal circuit is formed in an internal circuit forming portion of the channel forming region of the second semiconductor layer,
the output circuit is formed in an output circuit forming portion of the channel forming region of the second semiconductor layer, and
the preamplifier circuit forming portion, the internal circuit forming portion and the output circuit forming portion of each channel forming region of the second semiconductor layer are respectively enclosed on all sides by the conductor electrically separated from the channel forming region and fixed to the reference potential.
(5) In the communication module described in any of the means (2) through (4), the conductive layer is electrically separated from the channel forming regions of the second semiconductor layer by insulating layers reaching the insulating layer in the direction of the depth of the second semiconductor layer from the surface thereof.
(6) In the communication module described in the means (3), each of the input terminals is formed on the input circuit forming portion of the channel forming region of the second semiconductor layer.
(7) In the communication module described in the means (3), each of the input terminals is formed on the input circuit forming portion and each of the output terminals is formed on the output circuit forming portion.
(8) There is provided a communication module comprising:
a semiconductor chip in which channels for allowing signal converting means to convert current signals inputted from input terminals to voltage signals and outputting the same from output terminals respectively are arranged in parallel in plural form; and
wherein the semiconductor chip is comprised principally of a semiconductor substrate in which a second semiconductor layer is provided on a first semiconductor layer with an insulating layer interposed therebetween,
the signal converting means is formed in a channel forming region of the second semiconductor layer, which is defined for each channel,
the input and output terminals are formed on the channel forming regions of the second semiconductor layer with the insulating layer interposed therebetween, and
a conductor fixed to a reference potential is provided between the input terminals and the second semiconductor layer.
(9) In the communication module described in the means (8), a conductor fixed to the reference potential is provided between the output terminals and the second semiconductor layer.
According to the above means (1) through (7), since the amount of transfer of noise power developed through the substrate can be rendered small, channel-to-channel crosstalk can be reduced.
Further, since the channel-to-channel crosstalk can be reduced, the communication module can be increased in reliability.
According to the above means (8) and (9), since the substrate and the terminals can be separated from one another, the amount of transfer of noise power developed through the substrate can be lessened and hence channel-to-channel crosstalk can be reduced.
It is also possible to increase the reliability of the communication module because the channel-to-channel crosstalk can be reduced.